Automatic transmission fluids (ATF) are non-compressible lubricant compositions containing a number of conventional additives. As typically used, an ATF serves as a hydraulic fluid, activating and engaging gears in the transmission by a series of valves and other hydraulic circuits, and as a lubricant for the hydraulic pump used to provide hydraulic pressure for operation of the transmission. Engine oils are lubricating fluids containing conventional antiwear, antioxidant, and other additives in a mineral oil or synthetic oil base.
ATF, engine oils, and other functional fluids generally contain detergent and similar additives that tend to produce foam if air is entrained into the fluid. Additionally, impurities are produced in the fluid over time (for example by oxidation or degradation of the base oil), some of which may contribute to a foaming tendency in the functional fluid. Excess foam in a functional fluid can adversely affect its rheological, hydraulic, lubricating, and cooling performance. Entrained air in a hydraulic system fluid such as an ATF is a problem for the further reason that the air alternately expands in the low pressure inlet side of pump, and quickly contracts or is compressed as the fluid passes through the pump to the high pressure outlet side.
The resulting implosion of air bubbles on the outlet side causes pressure ripples in the hydraulic pump. The pressure ripples can damage the hydraulic system and can cause objectionable noise, manifested as “pump whine” in some transmissions. New automatic transmissions, such as continuously variable transmissions (CVT), with their compact sumps and high pump pressures, have raised the possibility of consumer reaction to the noise. In response, a number of OEM's have taken steps to reduce the air level in the fluid of their new transmissions by isolating or baffling the internal rotating components to separate them from the fluid, or by introducing aeration additives into the ATF to help the oil release the entrained air more quickly or otherwise reduce the level of entrained air. Additionally, conventional antifoam agents have been employed to help dissipate surface air bubbles.
The insolubility of the antifoam agents leads to some difficulties that must be addressed by the formulator of functional fluids such as ATF and engine oils. Typically, the antifoam agent is denser than the base fluids and tends to fall out during shipping and storage before being added to the transmission. In practice, this limits the amount of antifoam agent that can be incorporated or dispersed into the fluid by the supplier. Alternatively, a formulated fluid may be re-dispersed prior to use, but the extra step creates additional expense in the manufacturing process.
New methods for preparing functional fluids containing insoluble antifoam compounds are needed. It would be desirable to provide such methods for delivery of antifoam compounds into functional fluids such as transmission fluids and engine oils in order to provide advantages over prior art methods.